An electric power steering apparatus which provides a steering mechanism of a vehicle with an assist torque by means of a rotational torque of a motor, applies a driving force of the motor as a steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a PWM (Pulse Width Modulation) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Td of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of the steering torque Td detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 on the basis of a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value. Moreover, the steering angle sensor 14 is optional and it may not be arranged.
A CAN (Controller Area Network) 50 for transmitting/receiving various information about the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed V from the CAN 50. Further, a non-CAN 51 for transmitting/receiving communications, analog/digital signals, radio waves, etc. except for the CAN 50 can also be connected to the control unit 30.
The control unit 30 mainly comprises a CPU (also including an MCU, an MPU, or the like), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Td detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 (or from the CAN 50) are inputted into a current command value calculating section 31 for calculating a current command value Irefl. The current command value calculating section 31 calculates the current command value Irefl that is a control target value of a current supplied to the motor 20 based on the steering torque Td being inputted and the vehicle speed V being inputted and by means of an assist map or the like. The current command value Irefl is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm that a maximum current is limited, is inputted into a subtracting section 32B, and a deviation I (Irefm-Im) between the current command value Irefm and a motor current value Im being fed back, is calculated. The deviation I is inputted into a PI control section 35 for characteristic improvement of steering operations. A voltage control command value Vref that the characteristic is improved by the PI control section 35, is inputted into a PWM control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The current value Im of the motor 20 is detected by a motor current detector 38 and fed back to the subtracting section 32B. The inverter 37 uses FETs as driving elements and is comprised of a bridge circuit of FETs.
A compensation signal CM from a compensation signal generating section 34 is added in the adding section 32A, and characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further, adds the result of addition performed in the adding section 344 and a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
In such an electric power steering apparatus, friction caused by the reduction gears and the rack-and-pinion mechanism is large, and an equivalent inertia moment around the steering shaft caused by the motor for generating the assist torque is large. Therefore, in a low vehicle speed range that the self-aligning torque (SAT) is small, steering wheel returning gets worse due to large friction. Hence, since the steering angle does not return to the neutral position in the straight advancing state in the case only by means of the SAT, it is necessary to return the steering angle to the neutral position by steering intervention of a driver, as a result, this leads to the workload of the driver.
On the other hand, in a high vehicle speed range that the SAT is large, since the SAT is large, although a steering angle speed tends to become fast as compared with the low vehicle speed, since an inertia moment is large, an inertia torque is also large, as a result, the steering wheel does not converge at the neutral position of the steering angle and an overshoot occurs, therefore vehicle characteristics are felt unstable.
Accordingly, in order to assist the steering wheel returning in the low vehicle speed and in order to make the vehicle characteristics stable in the high vehicle speed, it is necessary to increase the convergence. Hence, in order to achieve those needs, various control methods for performing a moderate assist at the time of the steering wheel returning have been proposed. In these control methods of the steering wheel returning, there is an electric power steering apparatus disclosed in Japanese Patent No. 4685557 (Patent Document 1) as a prior art for performing a smooth steering wheel returning control even in the case of the steering intervention by the driver.
In the apparatus of Patent Document 1, a controller configured so as to follow a target steering angle speed, calculates the target steering angle speed by correcting a base target steering angle speed with multiplication and addition based on a vehicle speed and a torque. In the case of the steering intervention by the driver, the apparatus of Patent Document 1 reduces an uncomfortable feeling felt by the driver during steering by correcting the target steering angle speed in a direction to which the torque is applied.